Conventional electron-beam microlithography apparatus offer prospects of high-accuracy and high-resolution exposures but suffer from low throughput. Various technologies have been investigated in efforts to correct this fault. For example, certain pattern-portion batch exposure methods such as "cell projection," "character projection," and "block exposure" methods have received considerable attention. In a pattern-portion batch exposure method, small portions (e.g., 5 .mu.m square units of a pattern defining a memory portion) of an overall pattern (e.g., of an entire integrated circuit) that are repeated many times in the overall pattern are defined by respective regions on the reticle. The region on the reticle corresponding to the small portion is typically used repeatedly many times during the transfer of a die pattern to the substrate (e.g., semiconductor wafer) to form the overall pattern on the substrate. Portions of the overall pattern that are not repeated are typically transferred using a different method such drawing using a variable shaped beam. Unfortunately, such methods have very low throughput.
An electron-beam "reduction" (i.e., demagnifying) projection-transfer apparatus has been proposed that purportedly achieves higher throughput than pattern-portion batch exposure methods. In this type of projection-transfer apparatus, the reticle defines the entire die pattern (i.e., the entire pattern destined to be exposed onto a separate "chip" on the substrate). The pattern on the reticle is typically divided ("segmented") into multiple exposure units that are exposed sequentially by the electron beam onto the substrate. As the electron beam passes through an exposure unit, an image of the respective exposure unit is formed on a corresponding region of the substrate using a projection lens. The image is demagnified as projected onto the substrate, by which is meant that the image is smaller than the corresponding exposure unit as defined on the reticle.
In an attempt to improve the throughput of divided projection-transfer methods and apparatus, simultaneous irradiation of the entire reticle (i.e., "batch" exposure of the entire reticle defining an entire die pattern or even multiple die patterns) has been proposed. Unfortunately, such a technique exhibits poor transfer accuracy and poor edge resolution. It is also very difficult to produce a reticle that defines an entire die pattern (or multiple die patterns) to be transferred in one "shot" to the substrate.
Hence, divided projection exposure remains the favored technique for achieving projection exposure using a charged particle beam. According to one approach in divided projection exposure, the optical field of the projection-optical system is increased to allow projection of larger portions of the pattern during each shot. In any event, in divided projection-exposure methods, aberrations can arise during exposure of each exposure unit. Certain conventional divided projection-exposure apparatus achieve real-time correction of aberrations such as distortion or variations in the focal points of the images of the exposure units as formed on the substrate. Such corrections tend to improve the resolution and accuracy of pattern transfer over the entire die region compared to batch-transfer methods.
In exposure apparatus that employ a charged particle beam, exposed patterns can exhibit blurring (e.g., astigmatic blurring) and distortion. In a conventional variable-spot method or cell-projection method, each exposure unit is typically less than about 5 .mu.m square. In the conventional divided transfer methods and apparatus summarized above, the exposure units are typically larger, approximately 100 .mu.m square or larger (to increase throughput). With such large exposure units (each defining a respective portion of the overall pattern), if the features of the respective pattern portion are not evenly distributed, then the Coulomb effect can have a variable effect on image quality depending upon the distribution of pattern features in the exposure unit.
An example of an exposure unit having a non-uniform distribution of pattern features is shown in FIG. 5. In FIG. 5, the exposure unit 81 comprises multiple pattern features 87, 89. The features 87 are smaller than and spaced farther apart than the features 89. Also, the features 87 are congregated in a region 83 and the features 89 are congregated in a region 85. Hence, the feature density in the region 83 is lower than the feature density in the region 85. Each of the features 87 and 89 is defined on the reticle as an aperture (if the reticle is a stencil reticle) or a local region highly transmissive to charged particles (if the reticle is a membrane reticle). Hence, charged particles passing through any of the features 87, 89 apply a corresponding local dosage of charged particles on the substrate. (Such features are termed "positive" features.) The complementary portions of the exposure unit 81 tend to block transmission of charged particles and are termed "negative" features.
In FIG. 5, the higher-density region 85 within the exposure unit 81 has a feature density of 50% and the lower-density region 83 has a feature density of 10%. The local beam current of the beam passing through the higher-density region 85 will be higher than the local beam current of the beam passing through the lower-density region 85. As a result, the Coulomb effect will be more pronounced in the higher-density region 85. The differential impact of the Coulomb effect causes the point of best focus of the beam passing through the higher-density region 85 to be axially displaced relative to the point of best focus of the beam passing through the lower-density region 83.
Conventionally, transfer of the exposure unit 81 is performed at a "compromise" focal point for the regions 83 and 85. The compromise focal point, however, is not optimal for either of the regions 83, 85. This results in a corresponding decrease in overall resolution of the transferred image of the exposure unit 81 than if each region 83, 85 were exposed separately. The distortion in an image of an exposure unit 81 as projected is also different than any distortion in an image of an exposure unit with a more uniform feature density.